This invention relates generally to filament wound vessels and particularly to such vessels which are used in a state of ultra-high pressures.
Filament wound vessels often are constructed in a spherical shape for use in high pressure container applications. In many circumstances, the qualities of lightweight construction and high resistance to fragmentation and corrosion damage are highly desirable with such pressure vessels. Therefore, for some years, these design criteria have been satisfactorily met by the development of high pressure containers fabricated of laminated layers of wound fiberglass filament or various types of synthetic filament, such as nylon and rayon yarn, which are bonded together by a thermo-setting epoxy resin.
The spherical shape of high pressure vessels has other important advantages, including filament winding patterns which produce a quasi-isotropic composite. Such spherical composite pressure vessels currently are used for space shuttle tankage, for instance. In some applications, the vessels are required to contain ultra-high pressures, operating at 25,000 p.s.i. with design burst values in the 50,000 p.s.i. range. Composite pressure vessels are especially appropriate for such high pressure applications, since the manufacture of a thicker vessel wall structure essentially requires winding more layers of filaments. However, pressure vessels with thick walls are characterized by steep strain gradients through the wall, the inner strains being quite high when compared with those at the outer surface. An important parameter in controlling the strain gradient is the transverse or radial stiffness of the composite. Transverse stiffness is influenced by the wind angle of the vessel, as well as any delaminations or other defects induced during fabrication. Of major consideration, and thus a major problem, is the laminate void content of the composite. High quality laminates show a constant high value of transverse stiffness. However, if the laminate has voids or delaminations, such voids initially are crushed or closed, resulting in low initial stiffness.
The problem of laminate void content is particularly prevalent in polar wound vessels, such as spherical vessels or other vessels having a spherically shaped end. Most often, a polar opening is provided with a boss or fitting disposed in the opening and protruding outwardly therefrom. The vessel is wound by a known method in a numerically controlled winding apparatus using a sequence of polar winding patterns wound with incrementally increasing wind angles away from the polar opening or fitting. In practice, normally two layers of the first winding pattern are wound adjacent the polar fitting. Single layers of the remaining winding patterns are wound by increasing the wind angle in an incremental or stepwise fashion. The required thickness is obtained by repeating this sequence of patterns an integral number of times corresponding to the desired number of layers.
With such polar wound vessels, the region near the polar opening or fitting must be adequately designed to prevent premature failure in this area. More particularly, with such polar wound vessels, such as spherical vessels, the surface of the vessel resulting from a complete sequence of winding patterns, is irregular due to the high fiber build-up at the beginning of each pattern. This is most pronounced between the first and second patterns where the fiber path may actually be concave. When the next sequence of patterns is wound, the first pattern of the next sequence or layer of patterns must be wound over this surface and fiber bridging can result in air entrapment causing the aforesaid undesirable voids. There is a higher void area around the polar opening or fitting in the vicinity of the winding patterns of smaller wind angles.
The present invention is directed to solving the void problems mentioned above by providing a filament wound pressure vessel of the character described which includes filament filler means installed to fill the voids or delaminations and thereby prevent premature failure in the area near the polar opening or fitting.